The Population of Faint Optically Selected Active Galactic Nuclei at z ∼ 3

Abstract

We discuss a sample of 29 active galactic nuclei (AGNs) (16 narrow-lined and 13 broad-lined) discovered in a spectroscopic survey of ~1000 star-forming Lyman break galaxies (LBGs) at z~=3. Reaching apparent magnitudes of RAB=25.5, the sample includes broad-lined AGNs approximately 100 times less UV luminous than in most surveys to date covering similar redshifts and in the first statistical sample of UV/optically selected narrow-lined AGNs at high redshift. The fraction of objects in our survey with clear evidence for AGN activity is ~=3%. A substantial fraction, perhaps even most, of these objects would not have been detected in even the deepest existing X-ray surveys. We argue that these AGNs are plausibly hosted by the equivalent of LBGs. The UV luminosities of the broad-lined AGNs in the sample are compatible with Eddington-limited accretion onto black holes that satisfy the locally determined MBH versus Mbulge relation given estimates of the stellar masses of LBGs. The clustering properties of the AGNs are compatible with their being hosted by objects similar to LBGs. The implied lifetime of the active AGN phase in LBGs, if it occurs some time during the active star formation phase, is ~107 yr. Based in part on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA and was made possible by the generous financial support of the W. M. Keck Foundation.